Iary Davidzon

I got my Ph.D. at the Physics and Astronomy Dept. of the University of Bologna, working with Micol Bolzonella and Olga Cucciati on galaxy evolution topics. I studied the mass assembly and the star formation history of galaxies at intermediate redshift, relying on the spectroscopic information of the VIPERS survey.
At present, I'm postdoc at LAM working with Olivier Ilbert (and C. Laigle and H. J. McCracken in Paris). I exploit the exquisite multi-band data of UltraVISTA and SPLASH, to study the build-up of galaxies at z>4, i.e. in the first ~1.5 billion year of the life of the Universe. I also carry on some specific analyisis of very high redshift (z>7) candidates.
As a member of the COSMOS team I'm involved in several other projects, which are periodically updated here.
Together with Stefane Arnouts, I investigate the influence of environmental processes in the galaxy growth. The cornerstone of this project is the large scale structure reconstruction, in particular cosmic filaments, by means of the algorithm Disperse.
Sometimes I tried to take a day off to finish my work on the Shapley super-cluster, which I was doing during my Master's thesis under the supervision of Antonaldo Diaferio... but invariably I use such a free time to go bike riding or read on the couch...

VLT skyline

Research interests

click here for a complete list of publications

The past decade has seen significant advances in the study of galaxy evolution prompted by large astronomical surveys. They have been devised to conduct a census of star-forming and quiescent galaxies at different redshifts, by sampling wide portions of the sky and going deeper and deeper to span very large intervals of cosmic time.
My work is based on such large surveys, which collect large amounts of data thus facilitating statistical studies. The galaxy stellar mass function (GSMF) is one such fundamental statistic, which I extensively use in my research to trace the history of baryonic mass assembly and its conversion into stars for various galaxy types. In particular, the large volumes probed allow me to study rare massive galaxies. Moreover, observational GSMFs are pivotal to calibrate and/or test models of galaxy formation. Some of the scientific and technical questions that lead my work are:
- how does the high-mass end of the GSMF evolve?
- how to find elusive passive galxies at z>4? how did they stop forming stars?
- what is the role played by AGN at high redshifts?
- how to correctly identify high-z galaxies, and recover their physical properties?

Davidzon et al. 2017 (A&A, submitted).

We exploit the ultra-deep photometry of the COSMOS2015 catalogue (Laigle et al., 2016), which includes images from UltraVISTA-DR2, Spitzer, Subaru/Hyper-SupremeCam, and a wealth of ancillary data in the 2 sq.deg of the COSMOS field. Precise photometric redshifts have been tested with the unequalled spectroscopic sample of COSMOS (100,000 measurements): we find an error sigma_z=0.028(1+z) at z>3, for galaxies with [3.6]<25 mag. The increased UltraVISTA exposure time in the DR2, together with a panchromatic detection strategy, allow us to probe a stellar mass regime well below the characteristic mass even at high z. We derive the stellar mass function of COSMOS2015 up to z~6, i.e. over 13 billion years. In previous studies such a large redshift range have been studied only by combining GSMFs from different authors. By means of our coherent set of estimates, we are able to remove a few biases when studying galaxy evolution as a function of redshift.      
stellar mass function os COSMOS2015 galaxies

Davidzon et al. 2016 (A&A, 586, A23).

Thanks to the accurate spectroscopic redshifts of VIPERS, we are able to trace the 3-dimensional structure of the galaxy density field. We investigate its role in quenching the star formation at intermediate redshifts. We observe massive galaxies (ℳ > 1011) enhancing the stellar mass function of the overdense regions, with respect to the low-density ones (see plot). We devised a new method to normalise each mass function to the comoving volume occupied by the corresponding environment, so that we can relate estimates from different redshift bins and trace the GSMF evolution (faster in higher densities). We interpret these results through semi-analytical models and also with an empirical approach. The evolution of our low-density regions is described well by the formalism introduced by Peng et al. (2010, ApJ, 721, 193), and is consistent with the idea that massive galaxies become progressively passive because of internal physical processes. The same formalism could also describe the evolution of our mass function in the high density regions, but only if a significant contribution from dry mergers is considered. This is the first time data at z>0.5 are used to test the Peng et al. scenario.      
stellar mass function of VIPERS high- and low-density galaxies

Davidzon et al. 2013 (A&A, 558, A23).

We measure the evolution of the GSMF from z = 1.3 to z = 0.5 using the first public data release of VIPERS. Such estimates are publicly available on this page. Thanks to the large volume and depth of the survey, Poisson noise and cosmic variance of our GSMF estimates are comparable to the statistical uncertainties of large surveys in the local universe.
We determine with unprecedented accuracy the high-mass tail of the galaxy stellar mass function, which includes a significant number of galaxies that are too rare to detect with any of the past spectroscopic surveys. At the epochs sampled by VIPERS, massive galaxies had already assembled most of their stellar mass. We are able to separately trace the evolution of the number density of blue and red galaxies with masses above 1011.4, in a mass range barely studied in previous work (see the plot below). We find that for such high masses, red galaxies show a milder evolution with redshift, when compared to objects at lower masses. At the same time, we detect a population of similarly massive blue galaxies, which are no longer detectable below z = 0.7. These results give initial promising indications of mass-dependent quenching of galaxies at z ≃ 1.      
comoving number density of VIPERS red/blue galaxies